Add support for distance-between() or something similar

[yanokwa]

From https://groups.google.com/d/msg/opendatakit-developers/gQrS20_LMBc/GX25qPOcAwAJ

We have support for area() and it'd be nice to get something like distance-between().

[MartijnR]

I'm in favor. I just want to propose to name it distance() instead, unless people think there is scope for confusion.

Wrt its functionality, how about this description (based on how area() works):

Returns the distance in meters of either a nodeset of geopoints or a single geoshape value or a single geotrace value (not a combination of these) on Earth, in the sequence provided by the
points in the parameter. It takes into account the circumference of the Earth around the Equator and does not take altitude into account.

[lognaturel]

That sounds great to me, @MartijnR! I agree with distance() as the name.

[lognaturel]

And just to refresh my memory, we agreed not to namespace custom Xpath functions because XLSForms just passes them through, right?

[MartijnR]

I had to look that up as well. We left it here. Clearly we are bad, and are in trouble if official XPath specs use the same function name (for functions that do something else). However, it would be harder to shelter users from scary namespaces if we use them for XPath functions (would require more pyxform/etc magic to do - by adding them to the XForm output).

[meletis]

We have contributed the distance-between() and several other functions to ODK since last December but I can see those were never merged to the main branch:

https://bitbucket.org/m.sundt/javarosa/commits/7f3677c51d8c65f3ab285c542d6924f6d77775ae

I just remembered there was a public discussion with arguments whether those should be merged or not but I really don't remember what was the final result of this discussion. If I find it, I will post the link here.

[meletis]

Oh, thank you, @MartijnR. I saw your last comment after I had posted my comment above. :)

[muneneg]

Hi I am adding the area() in my xlsform, to calculate the area/size of a plot, the question type am using is geoshape. What unit of measure is my area() in, Acres/Ha/.
Thanks

[MartijnR]

It it's in square meters. See https://opendatakit.github.io/xforms-spec/#fn:area